Electron discharge apparatus



Sept. 2, 1947.

B. LLEWELLYN ELECTRON DI SCHRGE APPARATUS Filed Oct. 29, 1942 2 Sheets-Sheet l F/GJ l /NVE'NTOR f." B. LLEWELLYN BV v um a. aux;

ATT ORNE V Sept. z, 1947.l

vF. B. LLEWELLYN ELECTRON DISCHARGE APPARATUS '2 sheets-sheet 2 Filed 001;. 29, 1942 7. M G o r/f u m r. o o o o\l. ww um mr. uw Y AE unn ,o o 0 Olll m T u m m. m rm R0 mm l u Ar. /veoooowo oooooo.w\I w.\||\ooooooo MD Nm 0G C Vacuums afcrnoo:

cArHooE` GRID ATTORNEY Patented Sept. 2, 1947 ELECTRON DISCHARGE APPARATUS Frederick B. Llewellyn, Summit, N. J., assignor to Bell Telephone Laboratories, Incorporated,

New York, N. Y., a corporation of New York Application October 29, 1942, Serial No. 463,823

17 Claims. (Cl. 179-171) This invention relates to electron discharge apparatus and more particularlyA to electronic ampliers including electron discharge devices of the space charge virtual cathode type.

In the number of fields of use of electron discharge apparatus, for example in ampliers, it is highly desirable that signals within a wide band of frequencies be transmitted uniformly, faithfully and with high gain. A criterion for rating the eicacy of an electron discharge device as a transmission element is the figure of merit of the device, the ligure of merit being defined as the ratio of the transconductance to the sum of the control electrode or grid and anode capacitances.

Space charge virtual cathode devices oiier a possible means of lobtaining large values of transconductance, either positive or negative in sign. However, known devices of this type are characterized by instability so that the advantages of the high transconductance are counterbalanced in a large measure by the disadvantages of instability and critical fluctuations with operating conditions.

The two factors, namely transconductance and capacitance, determinative of the figure of merit of an electron discharge device are related and, in general, in devices of conventional construction, when the parameters involved are correlated to effect an increase inthe transc-onductance, this increase is accompanied by a subst-antially corresponding increase in the capacitance so that little, if any, effective change in the figure of merit results. l v.

One general object of this invention is to obtain a high figure of merit and stable operation for electron discharge devices. More specifically, objects of this invention are to:

Increase the transconductance of electron discharge devices without proportionately increasing the interelectro'de capacitances therein;

Obtain stable operation of high transconductance electron discharge devices yof the virtual cathode type;

Obtain stable negative transconductance of high value in electronvdischarge devices;

Facilitate the formation of a Vvirtual cathode in electron discharge devices having one or more space charge electrodes;

Substantially prevent feedback'from the anode to the control electrode in space charge virtual cathode devices suitable for use in high frequency and high gain circuits; and

Realize a high operating efciency for electron discharge devices of the space charge virtual cathode type.

In one illustrative embodiment of this invention, electron discharge apparatus comprises a device having a, cathode, an anode, a control electrode, and one or more positively biased auxiliary or space charge electrodes pervious to electrons mounted between. the cathode and the anode, appropriate potentials being applied to the various electrodes to result in the formation of a virtual cathode between the real cathode and the anode. The several electrodes may be substantially plane and parallel or cylindrical and coaxial.

In accordance with one feature of this invention, the several electrodes are so constructed and arranged that a substantially critical and appreciable portion of the electrons flowing in the device is collected or captured by the auxiliary electrode or electrodes, whereby very high values of transconductance with stable operation are achieved.

In accordance with another feature of this invention, the various electrodes are so constructed and arranged that less than a certain portion of the electrons are collected or captured by the auxiliary electrode `or electrodes, whereby stable operation with a high negative transconductance is realized.

In accordance with a further feature of this invention, the control electrode is biased at such negative potential and the auxiliary electrode or electrodes and anode are biased at such positive potentials that a potential minimum is established between two positive electrodes, whereby a virtual cathode is formed between these two electrodes.

In accordance with still another feature of this invention, the electrodes are cylindrical and mounted in coaxial relation with the anode the innermost and the cathode the outermost electrode of the structure, whereby the formation of the virtual cathode, with relatively 10W emission currents, is facilitated and relatively small loutput capacitance is obtained,

The invention and the above-noted and other features thereof Will be understood more clearly and fully from the following detailed description with reference to the accompanying drawing in which:

Figs. 1 and 2 are diagrams illustrating the potential distribution between two positive electrodes in a vacuum, for a number of space charge conditions;

Figs. 3, 4 and 5 are diagrams illustrating the potential distribution in electron discharge devices having one or more space charge grids and operated under such conditions that a virtual cathode is formed;

Figs. 6 and '7 are diagrammatic views of electron discharge devices illustrative of those constructed in accordance with this invention wherein the electrodes are substantially plane and parallel;

Figs. 8 and .9 .are sectional views-of electron-.discharge devices l'illustrative of those constructed in accordance with this invention wherein the electrodes are mounted in coaxial relation; and

Figs. 10, 11 and 12 are circuit diag-ramsshowing i illustrative manners of operating electron discharge devices constructed in Paccordar-ice' with this invention.

Referring to Fig. 1 of the drawing, if two Vplane electrodes I and 2, having positive potentials Aof V1 and V2, respectively, measured with respect to some reference axis X--X', are mounted in 'parallel relation in a vacuum and no electrons are present .between these electrodes, the potential distribution between the electrodes may fbe represented yby thestraight line A. If electrons are injected 'into the region between .electrodes I :and .f2 at :the plane of electrode v'I andwitha velocity in the direction of the X axis corresponding to vthe potential Vi, these electrons will lmove toward the electrode .2 and some vor .al1 ,of these electrons, depending :upon the rate at which electrons .are i inje'cted, *will be .collected by the electrode For a relatively small :injected current, the potential distribution between the electrodes may be represented by the curve .1B,. If -lthe'rate at which electrons lare thu-s injected, vlwith'lthe .same velocity, increased, the potential .distribution curve will sag as representedrbyithe curve `C.

.Ifthe rate of injection isinc-reased further, at some critical value, the .potential distribution curve will sagsharply and `:touch theX axis .astindicated by the curve D. If the rate of injection is increased :still further .beyond the .critical value, the 4point Eat which .the vcurve :Ditouches :the .X axis shifts to :the .left :or right in Fig. 1 in .such :a

Way `that .a varying number `of .electrons flow to the electrode' 4and the remainder :turn back and are collected by the relectrode '=.`I. Effectively, therefore, when the critical value of injected :current is reached, 4a virtual cathode is established in kthe immediate proximity vof 1a plane parallel to the electrodes'JI fand 2 and passing through the point 'where the curve D touches 4the X axis.

'The conditions necessary vfor the establishment of a potential minimum, and, fh'ence, :for Vthe formation .of a virtual cathode maybe determined from the following considerations, with reference particularly A:to Fig. .2. In 'this igure, curve D represents the 'potential `rdistribution whena virtual cathode has been formedXiand X2 are Ythe distances :between 'the electrodes YI .and 2, respectively, land the .point 'where 'the vcurve D touches the X axis, C is the fdistance .between the electrodes, I2 iis the fcurrent fowing :to :the electrode 2 from the virtual cathode'and I1 .is `thesum of the currents .carried by VJthe electrons originally injected :at the plane of .electrode .I and :that .carried by the Velectrons which flow from `the region of the vpotential minimum :back towerdithe Aelectrode I. Ii" Is is the injected current,..

`It can :be yshown that the general relationship requisite for the formation of a virtualicatho'de is 4 per square centimeter and potential in volts above the cathode from which the injected electrons emanate.

In order to obtain a high transconductance, it

is necessary that I2 be changed to a large extentwhen some other variable is changed to a relatively small extent. For purposes of general discussion, this variable may be designated as Z and the lfollowing variational relation is then Yobtained from Equation 2:

It will be understood that Z `may be any one of a number of factors. The three most important cases-.of control of I2 for practical purposes are:

1. Control by variation of the potential of a third electrode, e. g., a controlgrid,

2. Controlfby variations of Vaand 3. Control by variation o'f V1. Y

A typical arrangement .for the vfirstV case Yis shown in Fig. 3 wherein 'there isillustrated the potential distribution'in an electron discharge device comprising a cathode, an anode, a positively biased auxiliary electrode, a positively `'biased focussing or shielding electrode anda negatively biased control electrode arranged as shown, the auxiliary,` focussing and control electrodes .being perforate, e. g., grids. It may jbe .noted `that in Fig. 3 the portion D of the 4distribution Charactere istie -.corresponds Ato the curve DinlFig. f2.

If .the potential Vg o the control .electrode is considered as the variabileZ notedabovefEquation 3 may be writtenas v 5I s 'verlie/$41+Mw-mii =o to The quantity 1 2 5V, s f it will v.be noted, is 4the transconductance -of 'the device. shown byEquation 4,the value-of this transconductance (depends upon 'theA -factor a -l/.X the transconductance,

a ,l/'g Y Y is dependent upon an important effect ofthe grid structures' constituting the Iauxiliary and 'focussing electrodes,` namely, the capture .by these electrodes Aof a portion of theelectron current.

This factor and hence By constructing the device in accordance with'la feature of this '-invenltion, lless than a certain portion :of `the electrons are captured y.whereby highly desirable operating'characteristics and, .more par ticularly, desirable values of transconductance, for the device lare achieved..

The ei'e'ct =of the capture 'f 4electrons by "the grid structures will be -understood -f-rorn the Vfollowing considerations:'Let ,cibo the fraction of the electrons which the auxiliary V:andiocussingelectrodes allow ;to :pass .and Li the. electron :current :dowing .awayfrom the -vicinity .of'zthe cathode. the auxiliary and focussing electrodes is,`then,

(lun).V

Electrons .moving away from thewicinity io the cathode toward the potential zero lor virtual Y cathode region come underv the effect of the focussing and auxiliary electrodes. At theseielectrodes aI portion (1,-)I, of the electronsais captured and the remainder, ple, flow to thevirtual cathode region. From this region, an electron current I2 proceeds forward toward the anode and a current (Iv-Iz) ows backward toward the auxiliary electrode. Hence,- it will be seen that A portion of backward flowing current is captured bythe auxiliary and focussing electrodes. The capture effect of these electrodesupon the backward flowing electrons maybe assumed to be the same for electrons moving in either direction. Although thev assumption is not strictly correct, it is suiiciently accurate to; determine the effect of thecapture action of the electrodes and to indicate for practical purposeswhat design is to be employed. On the basis of the assumption noted, the portion of the backward current-captured which passes through the focussing electrode and iiows toward the cathode is ,Boele-J2). Hence, the total electron current Id in the vicinity of the cathode, both the forward and backward currents being considered, is

It is this current which produces the space charge near the cathode and which is controlled by the effective control grid potential in accordance with the well-known C'hilds equation.

From Equations 5 and 6, by elimination of Ic,

1 2 2 11= 51,311+ 2f I; 7) If, now, Vg is varied, it follows that y im wenn le@ a 7c-5V; 2s Wj" 2c )5V, (8)

gc, it will be noted, is the transconductance of a negative grid triode operating with as its anode potential, the equivalentpotential of the focussing electrode.

Solving Equation 8 for 5Vlz l substituting in Equation 4 and then solving for 6 12 5V.

the over-all transconductance is determined, to

wit:

is dependent upon the magnitude of the second termin the denominator on theright-hand side of EquationQ. That is, lwhen, this term is greater thanfunity, the transconductance is negative in sign and when this term is less than'unity, the

transconductance ispositive in sign. Generally, unless V2 is much less than V1, the transconductance is Ynegative .insign.x`V A, Whenthe term noted approaches unity, itwill be seen thatlthe,transconductance approaches iniinity. Theoreticallyjt would appear that ifcthis termwereincreased.progressively from a value lessthan unity to avalue greater than unity, the transconductance would pass through infinity and thenbecome negative insign. Actually instead of this occurring, when infinite valuesV of transcondutance-,are approached too closely, the potentiaidistribution jumps to one of its other pos- ,sibley Slflapes;and the. innite .,trarlsonductance does notv materialize.

It is important to note the effect of limiting Values ofthe capture fraction, When the capturel fraction; isverysmall, i. e., when substantially no electrons are captured by the focussing and secondary electrodes, ,e approaches unity and the-transconductance from which it will be seen that a negative transconductancepf greater magnitude than the triode transconductance is obtainable when V2 is so small in comparison with V1 thatvthe denominator in the right-hand side of Equation 10 is less than unity. For these conditions, the transconductance l is negative in sign, of large magnitudek and not subject to the sudden jumps in the formbf potential distribution previously noted when the two terms in the denominator of the right-hand side of Equation 9.are nearly equal.

The stable high negative transconductance is obtained by constructing the focussing and auxiliary grids of fine wires widely spaced and operating the device with the potential V2 at as low a value as is practical.

At the other Iextreme of the possible values of It is seen from an inspection of Equation 11 thatrthe transconductance may be positive or' negative in sign depending upon whether th'e second term in the denominator of the right-hand side of the equation is less than or greater than unity. This, in turn, depends largely upon the ratio Viz/V1 inasmuch as, as pointed out heretofore, I1/I2 always is greater lthan unity. Vz/Vi must be'lessthan unity in order that the transition point from positive to negative transconductance shall be approached. In th'e Vicinity of this transition point very large magnitudes of transconductance may be obtained inasmuch as the denominator of the right-hand side of Equation 11 is then very small. Such magnitudes-lare rather critical and Vary greatly with small changes in any of V1, V2, I1, I2 or and the high transconductances obtained in the manner indicated areA of use in cases Where the advantages of their magnitude outweigh the disadvantages of v:the .instability and fiuctuationswith 'opera-ting conditions associated therewith. 1 L

, Between the two extremefvalues of the capture fraction considered hereinabove, there is a range :of values `of for which the stable properties of transconductance 'attending operation with small values of and low values :of kVix/V1 may f-be .combined .with the very high 'transconducta-nces occurring when-.the two termso the denominator of Equation .9? approach equality. This Alatter condition requires that The less than a certain value ionas -noted heretofore, if A is too .lar-ge,V the. second term in the denominator of Equation 9 exceeds the first. Therefore, in order to obtain the `very high transconductance indicated to be realizable -by balancing the two termsi-n the denominator of-Equation 11;the focussing and auxiliary electrodes must bei con struc'ted `so that the capture fraction thereof is appreciable Wh'en 'the device is operated with the desired bias potentialsapplied to the electrodes.

It has been determined that the high transconductance and stable operation are obtainable if the focussing and auxiliary electrodes are constructed so that the capture fraction is approximately `0.2, that is, so that lis substantially 0.8. For a xalue 'for `of 0:8, Equation 9becomes i 51g- )4145516 p v fg-VL Vus/4 11 ls/z From Equation 12, it is apparent that very high values of the transconductance Vka will obtain when f l t Vlf/i'vli-.z/z L e is comparable with 4156. ,As noted heretofore, I2 always is less than I1 and, therefore, 'V1 must be made much greater than V2. VFor examplaif i Y 1;2 Y

the required ratio -of Vi/Vz would 'be 9.3 so that if Vr were 93 volts, V2 would be only 10 volt-s. In the case under-consideration, V2 is the anode potential and, thus, it will loe seen that a high transconductance 'is obtained jwith :ya lowY anode potential. Such low anode potential enablesY realization of high operating efficiencies.

In cases Where a low output impedance' for the device is not desired, a grid screen or shield electrode may be employed as the electrode providing the potential V2 and the. voutputzanode may be mountedsb'ehind the shield or .screen electrode were 0.9i'

' with reference to the' cathode..

tion .3, tit `can be :shown Vin a kmanner simil-ar to' that used .infarriying.atEquationQ that from which it will Abe appa-rentthat, as in the first l case lnoted .above,.very fhigh :values 1ofrtranscondenominator Vof the right-hand side approaches ductance are obtainable-ii the secondterm in the unityy and. vthat the #term inw-question Ydepends markedlyupon the capture fraction (`1-,)-. Such n very high values of transcon'ductancel are yobtain- .In .the l.'secondfcase.notedabone, the'tcurrentgz i to theiianode, iSv-varied :by chan'gi-ngthelpotential V2 4in Fig. 2; As show-n in Fig. v1i, .in 'a typical structure illustrative oi this case., the'positive auxi-liary electrode having the potential V1 is ,slfrielded iromithe cathode-:by .a perforated electrodewhich may serve also to 4focus 4the electrons from the cathodeV upon the openings .inthe positivel auxiliary electrode. The structure includes `also `a control grid positioned l:at 4a plane `of potential V2 between the auxiliary electrode and anodathe anode .having applied 'thereto -a potential ofthe order of that upon vthe auxiliary electrode.` Although, as lseen .in Fig. el, rvthe eective .potential of the tcont-rol,grid is slightly positive, the externalY bias applied .to the controlgridis-negatiye, inas-r muchasthe-effeetve Apotential :of raneeatire grid,y

able only if the focussing eiiiciency `fis not exs tremelylhighf soy that ydoes not approach unity too closely. Ityvill vbe noted 'fromv Equation "13 thatvthe Vtransconductance tends toward '3/2Iz/V2 ifthe focussing efficiency ifs high so that -Ver'y closely approaches unity.Vr value1-may be augmented by so construopting and arranging the electrodes that the-positive auxiliary electrode V1 collects an appreciable-portion;for example, ofthe Y order of 270 percent, of .the current flowing to the region thereof from the cathode.

For'practical purposesY it'Y is desirable that V2 be'inuchl smaller than V1 in order that thecontrol grid may be biased negatively; This requires,

as indicated by Equationj13, that'Izj-mustbe correspondingly 4less than l1 in order'thatfthe'second term of the denominator onf the Aright-hand Vside may approach unity.

If, as in the rst casais taken vas 0.\8.it,will

be apparent from Equation l'that very lhigh transcend-1ictances.v are' obtainableA when.

is .approximately equal,to4.56.r.` v Y l In the third case notedabove, co-ntroloi anode current I2 is effected .by varying Vi.' A typical structure for this case is illustrated in Fig. 5 and comprises a cathode, an anode, a positive screening and focussing grid and a control grid, the latter being mounted in aY plane corree spondingrto a potential V1. It will be noted that the relative position of the electrodes in Fig. 5 is similar to that in Fig. 4 With the difference that. whereas in 4 ther-spacing between the positive auxiliary electrode and the control grid 'is large in order to allow the formation ofthe virtual cathode with moderate values of currents, in Fig. 5 the spacingV lofjthe control grid and anode is large-for the same reason.

Considering V1 asthe variable Z in Equation 3, for this ithird. case it can 'beshown that Athe transconductance is .given by the relation:` Y v where g is the change-inthe sum of outward Typical devices constructed in accordance with .this invention are illustrated in Figs. 6 and '7 wherein the several electrodes are substantially plane and parallel. The control, focussing and auxiliary electrodes are formed of parallel Wires,

corresponding wires of the several electrodes being aligned. These wires may be circular, as illustrated in Fig. 6, of very small diameter and Widely spaced to provide a capture 'fraction of substantially 0.2 for the focussing and auxiliary A in turn is determined by the density of electrons.

A high electron density and, hence, a ready formation of the virtual cathode may be obtained with relatively low emission currents from the actual cathode if the area of the electrode having the-'potential V2 is less than that of the electrode having the potential V1.

Illustrative structures for facilitating the formation of a virtual cathode with relatively low emission currents are shown in Figs. 8 and 9. In the structure shown in Fig. 8, the electron discharge device comprises an evacuated enclosing vessel I0, a central linear rod anode II and a cathode I2 comprising a plurality of equally spaced linear elements parallel to the anode II and mounted in a cylindrical boundary coaxial therewith, The cathode elements may be iilaments connected in series or parallel or they may be equipotential indirectly heated members. A1-

ternatively, the cathode may be a cylinder coaxial with the anode. indirectly heated and having the inner surface thereof coated with a thermionic material.

.Mounted between the cathode and anode and coaxial therewith are a plurality of cylindrical grid electrodes I3, III and I5 each of which comprises a plurality of equally spaced, linear elements parallel to one another and to the anode I2. Corresponding elements of the several grids I3, III and I5 may be readily aligned or not so aligned.

In structures operated as described hereinabove for the second and third cases, it is Often important that the anode be screened from the control grid to prevent feedback in high frequency and high gain circuits. This may be effected, as illustrated in Fig. 9, by providing a grid electrode I5 in proximity to the anode, this grid electrode comprising, for example, a plurality of equally spaced linear elements parallel to one another and to the anode and mounted in a cylindrical -boundary coaxial with the anode.

In the structures illustrated in Figs. 8 and 9, it will be noted that the output capacitances may be made relatively small because of the small areas of the electrodes, particularly of the anode, and of the relatively large spacings employed. Hence, inasmuch as the gure of merit is inversely dependent upon the interelectrode capacitances, these structures not only facilitate the formation of a virtual cathode relatively remote from the actual cathode but also enable the attainment of an increased ligure ofv merit by reducing the interelectrode capacitances.

A circuit illustrating one manner of utilizing devices of the constructions shown in Figs. 8 and 9 is shown in Fig. 10 which illustrates operation for the second case described above. The anode II is connected to the cathode I2 through the primary Winding of an output transformer I'I and is maintained at a positive potential, for example if the order of 100 volts, with respect to the cathode by a source such as battery I8. The grid I5, which serves as a -screening and focussing electrode, is Vmaintained at a positive potential, for example, of the order of 10 volts, and the gird I4, which serves as an accelerating electrode, is maintained at a positive potential somewhat lower than that of the anode, for example, of the order of volts. The grid I6 is maintained at a positive potential, for example, of the order of 80 volts. The grid I3, which serves as the control electrode, is connected to the cathode through the secondary winding of an input transformer I 9 and has applied thereto a negative bias, for example, of the order of 5 volts, by a source such as a battery 20.

In asimilar circuit illustrated in Fig. 11, for operation for the first case described above, the grid I5 serves as the control electrode and is connected tc the cathode through the secondary Windingof transformer I9 and negatively biased by the battery 20 and grid I3 serves as the accelerating electrode and is connected to a positive point on the battery I8.

In a similar circuit, illustrated in Fig. l2, for operation for the third case noted above, the grid I3 is omitted and the grid Il! serves as the control electrode and is connected to the cathode through the secondary winding of the transformer I9 and biased negatively by the battery 2l).

Although specilic embodiments of this invention have been .shown and described, it will be understood that they are but illustrative and that various modiiications may be made therein without departing from the scope and spirit of this invention as deiined in the appended claims.

What is claimed is:

1. Electron discharge apparatus comprising a pair of electrodes mounted in spaced relation, one of said electrodes being apertured, means including a cathode and a focussing electrode for inj ecting an electron stream through said apertured electrode into the space between said pair of electrodes, means biasing said pair of electrodes at positive potentials with respect to said cathode such that a virtual cathode is established between said pair of electrodes, and means for controlling the current from said virtual cathode to the other of said pair of electrodes, said one electrode and said focussing electrode being grids having a capture fraction of the order of 0.2.

2, Electron discharge apparatus in accordance with claim l wherein the bias applied to said one electrode is large compared to the bias applied to said other electrode and wherein said controlling means comprises a negatively biased control electrode between said cathode and said one electrode.

3. Electron discharge apparatus in accordance with claim 1 wherein said controlling means comprises a control electrode positioned adjacent the region of said virtual cathode and biased negative with respect to said cathode.

4. Electron discharge apparatus comprising a pair of electrodes mounted in spaced relation, one of said electrodes being a grid formed of ne 'wires widely spaced whereby its capture fraction is very small, means including a cathode and a agee-,eac

focussing electrode for projectingr an electron stream through said grid' intothe space between said pairs of electrodes, said focussing electrode being a grid formed of fine wires widely spaced, a Vcontrol electrode between said cathode and said focussing electrode, and means applying positive potentials to'said pairV of electrodes such that a virtual cathode is formed between said pair 1 of` electrodes, the potential applied tosaid one electrode being greater than the potential applied toV the other of said pair of electrodes, and the potentialapplied to; said other electrode` being as small ets-practicalv whereby the quantity is negative in sign, I2 being the currentA 4to said i other electrode and Vg being the potentialv of said control electrode.

5. Electron discharge apparatus comprising a i cathode, an electrode spaced from said cathode,

a control electrode between said cathodeand said first electrode, auxiliary electrode means between said cathode and said rst electrode, and means applying a positive potential to said auxiliary electrode means and a smaller positive potential to said firstelectrode, such that a potential minimum is established between said auxiliary electrode means and said first electrode, said auxiliary electrode means having a capture fraction of substantially 0.2.

6. Electron discharge apparatus comprising a cathode, an electrode spaced from said cathode, a control electrode between said cathode and said first electrode, auxiliary electrode means between said cathode and said iirst electrode, means applying a positive potential to said auxiliary electrode means and a considerably smaller positive potential to said rst'electrode such that a virtual cathode Vis established between said auxiliary electrode means and said rst electrode, said auxiliary electrode means having an appreciable capture fraction andV said positive potentials being related so that the quantity approaches unity, where V2 is the vpotential of said auxiliary electrode means, V1 is the potential of said rst electrode, Ii is the total electron cure rentin the .space between said auxiliary electrode means andthe region of said potential minimum, I; isthe current flowing to said rst electrode and (l-) is said capture fraction.

'7. Electron discharge apparatus comprising a cathode, a iirst electrode spaced from said cathode, a control electrode between saidcathode and y said rst electrode, auxiliary electrode means between said cathode and said first electrode including a grid composed of fine wires widely spaced such that the capture 'fraction thereof is very small, and means applying positive potentials to said first electrode andV said auxiliary electrode means, such that a virtual cathode is formed between said rst electrode and said. electrode means, the potential applied to said rst electrode being small in comparison with the potential applied to said auxiliary electrode means suchthat the quantity is. lessthan. unity, where. V2 is the potential. ofV

saidfirst. electrode, V1 is the potential of. said that'. a virtual cathode isv established .between said. grid means and said 'rst electrode, said .grid

means having a capture fractiorrlofi substantially 0.2, and the. potential orsad grd..means.f being of thel orderofv .ten times-'the potential o'said iirst electrode. f

9. Electron discharge apparatus. comprisinga cathode, an anode, a pair of grids in spacedrelation between `said cathodejandi saidanode, an output circuit connected to said .cathode and said anode, an input circuit connected tosaid cath,- ode and one of said' grids,v andmeans applying such positive potentials toY said anodey andd the other of saidV grids that a Ypotential minimum obtains between saidi anode and said'other grid,

said other grid being so.y constructed' and are ranged that it collects of the order ofi 20. percent. ofthe electron current. flowing from said cathode toward saidanode.

10. Electron discharge apparatus in. accordance with claim 9, wherein said one grid is mounted between said. anode and saidother grid andin proximity to the: region atV whichV the potential minimum obtains'. Y f

11. Electron discharge apparatus in accordance with claim 9 wherein saidi one grid isvmounted between said cathode and `said :other grid'.

12. Electron discharge apparatus'in. accordance with claim9 wherein said' other' gridf is "of an area greater than'they area of said anode.

13'. Electron discharge apparatus comprising an anode, a cathode encompassing said anode, a ypair Y of spaced grids between said anode' and' cathode andencompassing said anode, an output circuit connected to said cathode and said' anode, an input circuit connected to said` cathode and one of said grids, and means for applying such .potentials to -said grids and said anode thatapotentialV minimum obtains inthe region between said anode and the grid nearest thereto.

14. Electron discharge apparatus in accordance with claim `13: wherein. said one grid is between said cathode andthe other-of saidA grids and said otherV grid is maintained at apositive potential several times. as great as the anode potential.

1'5. Electron discharge apparatus in accordance with claim 13 wherein said one gridis between said anode and the otherL of said grids and in proximity to the regionv at. which the potential minimum obtains'. Y

16. Electron discharge apparatusv comprising a linear rod anode, a. cylindrical cathoder encomy passing and coaxial with said anode, a. pair of spaced cylindrical. grids between said 'cathod'eand said anode and coaxial therewith, an outputv circuit connected between said cathode and said anode, an input circuit connected betweenV said cathode, and one of. said grids and: including meansA forapplying. a negativev bias; to` said one grid, and means maintaining saidanode-xand the other of Y saidjgrids at.- positive potentials:with` re.- Spect tosaid` cathode., 'said vlmtelltials;.and bias being such that a potential minimum. obtains REFERENCES CITED The following references are of record in the le of this patent:

Number UNITED STATES PATENTS Name Date Thompson Dec. 27, 1938 Farnsworth Oct. 2, 193'4 Jobst Sept. 24, 1935 Herold Jan. 7, 1941 OBrien Jan. 31, 1939 Herold Dec. 31, 1940 Peterson Oct, 8, 1940 Bruce Mar. 12, 1940 

